1. Field of the Disclosure
The present application relates to a solid state laser that generates deep-UV light (such as near 193 nm in wavelength) by harmonic conversion of an infra-red fundamental wavelength. Such a laser is suitable for use in photomask, reticle, or wafer inspection.
2. Related Art
Shorter wavelength laser light can produce higher resolution images, which in a laser inspection system can provide better information regarding features and defects on the imaged samples. To meet the increasing demand for laser inspection systems having ever higher resolution, the current trend in the semiconductor industry is toward the development of short wavelength UV-DUV laser inspection systems (i.e. systems utilizing laser light below 250 nm). For example, short-wavelength UV-DUV laser inspections systems operating with 213 nm, 206 nm, or 193 nm laser light are currently being developed.
To minimize the cost and complexity required to generate an optical system for a short wavelength UV-DUV laser inspection system, an illumination source must be able to generate laser light in which substantially all of the light energy is within a narrow bandwidth. In UV-DUV laser inspection systems, the bandwidth range at which 95% of the energy is contained (i.e. the light's “E95” bandwidth value) is generally the desired goal. Therefore, the challenge is to provide an illumination source that generates narrow band UV laser light that is both short wavelength UV-DUV (e.g. light having a nominal wavelength value below 250 nm) and has a narrow E95 bandwidth (i.e. within ±1%, and preferably within ±0.1%, of the nominal or “central” UV frequency).
There are two types of solid state lasers typically used in the generation of narrow band UV light: bulk lasers and fiber lasers. Bulk lasers include an active solid medium of glass or another crystalline material that is doped with rare earth elements, such as neodymium, chromium, erbium, or ytterbium. Bulk lasers can produce laser light having very narrow bandwidths and high peak power, which allows for the use of less complex (and therefore lower cost) optical systems. However, the wavelength choices for bulk lasers are very limited and thus are not suitable for some laser inspection systems. Moreover, generating reliable high power light from a bulk laser is challenging.
In contrast to bulk lasers, fiber lasers include an active gain medium formed by an optical fiber doped with rare-earth elements, such as erbium, ytterbium, neodymium, dysprosium, holmium, praseodymium, or thulium. Fiber lasers are an attractive choice for generating fundamental light in laser inspection systems because they can generate laser light having high peak power. Moreover, the frequency of the laser light can be “tuned” to a specified frequency by altering the amounts of doping materials in the fiber(s).
FIG. 1 illustrates a conventional fiber-based illumination source 100, which can generate UV laser light for an inspection system. Fiber-based illumination source 100 has a master oscillator power amplifier (MOPA) configuration that includes a seed laser 101 and a fiber amplifier 105 to boost the output power. Although a MOPA configuration is more complex than a bulk laser that can directly generate the required output wavelength and power, its constituent components are generally off the shelf and therefore may be simpler to develop than a new bulk laser with higher output power.
For example, in this embodiment, fiber-based illumination source 100 includes a seed laser 101 that outputs pulsed light, e.g. at 1060 nm. An optical isolator 102 receives the pulsed light output and ensures that its transmission is in only one direction. Specifically, optical isolator 102 uses a Faraday rotator and its associated polarization to prevent unwanted feedback. An optical coupler 103 receives the polarized output of optical isolator 102 as well an input from a pumping light source 104. Pumping light source 104 is used to transfer energy into the gain medium of fiber amplifier 105. This energy is absorbed by the medium, thereby exciting states in its atoms. In typical embodiments, the pump energy can be provided by an electric current or light. However, in either embodiment, the pump power is higher than the lasing threshold of seed laser 101.
A fiber amplifier 105 receives the output of optical coupler 103 and provides power amplification to the energized, pulsed light. In one embodiment, fiber amplifier 105 includes one or more ytterbium-doped fibers (YbDFs). An optical isolator 106 can receive the amplified, pulsed light and eliminate feedback, as described above. Note that a MOPA configuration can be sensitive to back-reflection, particularly after light amplification. Therefore, optical isolators (e.g. optical isolators 102 and 106) can include a Faraday isolator to mitigate this feedback sensitivity. An optical filter 107 can receive the polarized output of optical isolator 106 and generate an output light 108. In one embodiment, output light 108 can include one or more wavelength components (i.e. fundamental light sources). When multiple wavelength components are present, additional components, such as switches, can be used to select the desired wavelength component. In one embodiment, additional amplification stages including optical isolators, pumping light sources, optical couplers, fiber amplifiers, and optical filters can be included in fiber-based illumination source 100.
Unfortunately, each additional amplification stage adds complexity, especially at high average and peak powers. At average power levels of 40 W and peak powers of 20 kW it is very difficult to splice optical fibers so they will not damage. In addition, active cooling of the fibers and connectors becomes necessary. High power amplifiers also require increased pump powers adding to the heat generation. Pulsed sources also cause self-phase modulation (SPM) which will increase the spectral bandwidth of the laser. This places fundamental limits on how much average and peak power can be extracted from a fiber amplifier. Therefore, a need arises for an improved illumination source.